There have been many improvements in the combination portions of combination locks to make them more secure. However, the improvements have been mainly directed to improving the combination mechanism so that the lock resists manipulations to ascertain alignment of tumbler gates. Normally, the manipulations are either by sound or feel, and one attempting surreptitious entry into the safe attempts through certain techniques to dial the lock combination.
Manipulative techniques using either sound or feel are difficult and time consuming. Many attempt to physically enter the lock housing in order to gain access to the tumblers and the linkage to the bolt. The most common methods are either by drilling or by use of an acetylene cutting torch. For example, it is a common technique to cut into the housing to reach the linkage between the tumblers and the bolt. By moving the linkage, the bolt can be withdrawn into the housing and the safe can be opened.
One of the objects, therefore, of the present invention is to disclose and provide a combination lock which can be prevented from being unlocked when the case is entered by means of drilling or acetylene cutting. Specifically, one object of the present invention is to disclose and provide a mechanism which will lock the lock bolt independent of the tumblers when the housing is subjected to a predetermined temperature which would occur from an acetylene torch or from friction created during drilling. A further object of the present invention is to disclose and provide an additional locking mechanism which is difficult to tamper with even if partial access is gained to the inside of the cse. Still another object of the present invention is to disclose and provide such an additional locking mechanism that is easily assembled inside the lock housing, and which can be easily set when the housing is assembled.
Of course, it is an object of the present invention that the additional locking mechanism be both reliable and of low cost. The present invention meets the previously stated objects, and it also meets other objects that, although not specifically listed as objects, are evident from the description of the invention.